Cyanides are widely employed in the electroplating industry to help maintain high concentrations of metallic ions, such as zinc and cadmium ions, in solution. The source of cyanide ion for such solutions is often sodium cyanide. When the electroplating solution is rinsed from plated parts the waste rinse water generally comprises a dilute aqueous solution containing free cyanide and metal cyanide complexes. For example, if the metal is zinc, the rinse solution may include CN.sup.- anions and Zn(CN).sub.4.sup.2- ion complex. Spent electroplating solutions, or baths, contain generally the same ions but in much higher concentrations.
Problems have arisen with respect to the disposition of the plating waste solutions, i.e. rinse waters and baths. Upon exposure to acid, such solutions may generate hydrogen cyanide (HCN), a highly toxic and volatile material. In addition the heavy metal ions are themselves toxic. Both cyanide and metal concentrations are regulated by the government.
A typical plating rinse water includes a cyanide ion concentration of about 50 milligrams/liter (mg/l). Should such a solution be exposed to acid, a substantial amount of HCN may be formed. As a result, industrial and/or government safety standards have generally required that the plating waste solutions be treated to reduce cyanide concentration to less than about 1.0 mg/l, prior to discharge to municipal sewer systems. Purification is even more important if a concentrated bath is to be discharged.
A typical plating waste solution may also include substantial amounts of metal ions therein. For example, a waste rinse water from zinc plating often includes a zinc ion concentration of about 50 mg/l. Although such a solution is relatively dilute in zinc concentration, disposition of large amounts of such solutions leads to loss of a considerable amount of potentially useful metal. Further, the discharge of zinc is regulated under government pollution standards and zinc concentration in industrial discharges typically must be reduced to a concentration of less than about 2.6 mg/l for discharge to municipal sewers.
As a result of the above, attempts have been made toward: treatment of cyanide-containing waste solutions to reduce cyanide and metal concentrations therein to 1 mg/l or less; and, recovery of dissolved metals, such as zinc, cadmium or nickel, from such solutions for recycle and reuse. Metal recovery methods have generally involved the use of cation exchange columns to concentrate the metal cation. The metal cations are then rinsed from the column into a concentrated solution, for recovery by conventional means such as electroplating or electrowinning.
Cyanide (CN.sup.-) removal has been of substantial concern. In the plating industry, CN.sup.- removal is commonly achieved by the destructive process of alkaline chlorination. The plating waste is treated with sodium hydroxide and is chlorinated. The process generally involves the following three chemical steps: ##STR1## The first step of the chlorination process is instantaneous. The reaction of cyanogen chloride (CNCl) to form NaCNO is slow below pH 8.0, however it goes relatively rapidly at higher pHs. For example, at pH 8.5 conversion is complete in about 30 minutes. The last step concerns oxidation of the cyanate to carbon dioxide and nitrogen. The chlorine and sodium hydroxide requirements for this process are dictated by the stoichiometry of the above reactions.
Alkaline chlorination is relatively expensive, due to the large chlorine and caustic demands. Further, the reaction leads to complete loss of cyanide; i.e. the cyanide is not recovered, concentrated and reused in further electroplating.
Anion exchange resins have been employed for the removal of cyanides from dilute rinse waters or waste waters. Generally, the cyanides are concentrated by means of a strongly basic anion exchanger. Problems have arisen, since regeneration of the exchanger may be difficult. A reason for this is that the rinse water contains not only free cyanides but also metal-cyanide complexes, for example Zn(CN).sub.4.sup.2-. Such cyanide complexes are often held very tightly by strongly basic anion exchange resins. Thus, it can be difficult to regenerate the ion exchange resin completely, and the resin gradually loses exchange capacity. In time the reduced ability of the resin to remove zinc and cyanide necessitates replacement of the resin. Also, disposition of the spent resin containing the complex therein has been a problem. Further, the costs may be prohibitive, in terms of spent anion exchange resin. To the extent that the resin is regenerated, disposition of regenerant, having zinc and cyanide ions therein, has been a problem.
The use of a cation exchanger and a weakly basic anion exchanger, ahead of a strongly basic resin, can reduce these problems. However, the method has not been completely satisfactory.
It is known that certain metal cyanide complexes such as Zn(CN).sub.4.sup.2- can be released from strongly basic anion exchange resins through the application of substantial amounts of sulfuric acid solutions thereto. In the past, there have been two basic problems with this approach to metal recovery, making it impractical for industrial use. The first is that large amounts of sulfuric acid (H.sub.2 SO.sub.4) have been required, generating cost and handling problems. Further, large amounts of HCN, in solution, are generated by such a process. That is, the sulfuric acid rinse causes formation of HCN, so that even with neutralization of the H.sub.2 SO.sub.4, the solution is not desireable. The generation of a large volume of solution having a substantial concentration of HCN generates considerable safety and environmental concerns. Also, the cost of caustic which would be necessary to neutralize such a solution, before disposition, further adds to the impracticality of such a process.
What has been needed has been a method for recovering cyanide from waste plating solutions which is efficient, relatively inexpensive and which, preferably, leads to the generation of a concentrated sodium cyanide solution that may be cycled back into the plating solutions, returning the CN.sup.- thereto. Further, a method has been needed by which metal recovery may be readily and inexpensively accomplished. It is an object of the present invention to provide a method accommodating both goals.
As described below, the method makes use of a microporous membrane. Microporous membranes have been utilized, in numerous industries, in a variety of manners. Such membranes are generally formed from a polymeric resin material such as polyethylene, polypropylene, polytetrafluoroethylene, polyphenylene oxide, polybutylene, polystyrene, polyvinylchloride, acrylonitrile-butadiene-styrene terpolymer, styreneacrylonitrile copolymer, styrene-butadiene copolymer, polysulfone, poly(4-methyl-1-pentene), polyvinylidene chloride, and chlorinated polyethylene. Materials which can pass through the microporous membrane are, in application, allowed to diffuse or filter therethrough for isolation.